Such pumps or hydraulic motors are known as bent axis pumps or motors. The plungers of the known pumps or motors are swivable connected to a flange and are movable in cylinders, which are at one end of a rotor. At the other end of the rotor a port plate is positioned; this end of the rotor forms the valve surface. The port plate is located between the valve surface of the rotor and the housing. In the known pumps or motors, the port plate positioning drive comprises hydraulic actuators, which move a coupling pin in a slot in the housing. The coupling pin is positioned in a hole in the centre of the port plate so coupling the port plate to the hydraulic actuators.
This known construction has the disadvantage that in the centre plane at the location of the slot the housing does not support the port plate sufficiently so that the port plate can deform under influence of the high pressure between the port plate surface and the valve surface. Also between the pressure ports, which is in the area of the centre plane, the pressure between the port plate surface and the valve surface fluctuates with the passage of the cylinder channels and thereby causes fluctuations in the deformations. It is not possible to compensate for these fluctuations in the design of the parts. These fluctuating deformations create gaps, which cause leakage of oil. If the deformations are limited, for instance to a maximum of 3 to 5 micro millimeters, the leakage between the port plate surface and the valve surface remains acceptable. A higher value reduces the efficiency of the pump or motor in an undesirable way. This requirement limits the first radius, as a larger radius reduces the stiffness of the port plate and increases the deformations.
A further disadvantage of the known construction is that it is not possible to extend the drive shaft through an opening in the port plate. Such an extension would make it possible to connect several pumps or motors inline. An opening in the port plate with a diameter suitable for letting the drive axis pass through would further reduce the stiffness of the port plate and would interfere with the hydraulic actuators.